Enhanced Solvent Vapour Extraction processes in Thin Heavy Oil Reservoirs
Solvent-based techniques, such as solvent vapour extraction (VAPEX) and cyclic solvent injection (CSI), have emerged as promising processes to enhance heavy oil recovery. However, there are still a number of technical issues with these processes, such as the theoretical modeling and performance enhancement. This thesis aims at addressing the following major technical topics. Theoretical modeling of VAPEX. Heavy oil−solvent transition zone is where the VAPEX heavy oil recovery occurs. Existing analytical VAPEX models can neither fully characterize the transition zone nor accurately predict its growth. Numerical simulation models use grid sizes that are much larger than the transition-zone thickness (~1 cm) and thus cannot capture the characteristics of the transition zone. This study develops a new two-dimensional (2D) mathematical model for the VAPEX process on the basis of its major oil recovery mechanisms (i.e., solvent dissolution and gravity drainage) inside the transition zone. This VAPEX model is able not only to accurately describe the distributions of solvent concentration, oil drainage velocity, and diffusion coefficient across the transition zone, but also to predict the evolution of the solvent chamber. Theoretical modeling of the diffusionconvection mass transfer in CSI. CSI is a solvent huff-n-puff process. One of the differences between CSI and VAPEX is that the operating pressure is decreased and increased cyclically in CSI. Hence, in addition to molecular diffusion, CSI has another mass transfer mechanism, convection, which is attributed to the bulk motion of solvent caused by the pressure gradient between the solvent chamber and untouched heavy oil zone. This study develops a convection−diffusion mass-transfer model for the heavy oil−solvent mixing process of CSI. The diffusion coefficient and convection velocity are both considered as variables rather than constants. Results qualitatively show that pressure gradient can greatly enhance the mixing process. Enhancement of VAPEX and CSI. This study proposes a new process, namely foamy oil-assisted vapour extraction (F-VAPEX) to enhance the VAPEX performance. F-VAPEX combines merits of VAPEX (continuous production) and CSI (strong driving force) together. It is essentially a VAPEX process during which the operating pressure is cyclically reduced and restored. It is found that the foamy oil flow during the pressure reduction period can effectively move the partially diluted heavy oil toward the producer. Results show that F-VAPEX can increase both the average oil production rate and the ultimate oil recovery of VAPEX. In comparison with CSI, F-VAPEX has a higher oil production rate and a lower solvent−oil ratio. This thesis also proposes a new process to enhance the performance of CSI, namely gasflooding-assisted cyclic solvent injection (GA-CSI). GA-CSI uses dedicated solvent injector and oil producer to prevent the ‘back-and-forth movement’ of foamy oil inside the solvent chamber during the conventional CSI process. GA-CSI applies a gasflooding slug immediately after the pressure depletion process of CSI to produce the partially diluted foamy oil left in the solvent chamber. It is found that the motionless foamy oil due to pressure depletion and solvent liberation serves as a buffer zone, which effectively reduces the mobility ratio between the displacing solvent and the displaced oil and leads to a high sweeping efficiency. In comparison with the conventional CSI process, the GA-CSI process can increase the oil production rate by over 3 times and in the meantime decrease the solvent−oil ratio from ~4 to ~3 g solvent/g oil.